Conventionally, silica gel is known as a moisture absorbent material (drying material) furnished with an indicator function. The silica gel itself is a white material and is mixed with cobalt chloride to indicate a moisture absorbent state in a manner to show a blue color when dried and a pink color when absorbing humidity (moisture) in the atmosphere.
However, under the present situation, the twenty first century is said to be the century for preserving the environment and energy resources, and it is desirable to avoid use of heavy metals such as cobalt as much as possible in view of an environmental effect thereof. Therefore, the development of a material with an indicator function has been promoted in an effort to stop the use of cobalt.
For example, although a moisture absorbent function is not provided, a material with an indicator function is known which includes a pH indicator and an alkaline material to utilize a change in color due to evaporation of the alkaline material, for example (see patent documents 1 and 2).
A material is also known which includes a color-changeable pigment and an alkaline material to utilize a change in color due to oxygen or humidity in the atmosphere (see patent document 3).
A material is further known which includes a pigment and an organic solvent to utilize coloring due to volatilization of the organic solvent (see patent documents 4 and 5) or which utilizes a photochromic substance (see patent document 6).
The following invention is proposed for an environment determination indicator.
A temperature-indicating ink, a pH indicator, a photochromic ink or the like is used for a color-changing layer to correspond to environmental changes such as temperature, humidity, ultraviolet radiation amount, ammonia concentration, pH value, etc. Due to environmental changes, the color-changing layer changes from a certain color to a colorless one, or from a light color to a deep color, to thereby exhibit the indicator function (see patent document 7).
Patent document 8 describes a humidity indicator using a dye that absorbs moisture to change color.
Patent Document 1: Japanese Patent Application Laid-Open Publication No. 56-131684 (entire description)
Patent Document 2: Japanese Patent Application Laid-Open Publication No. 62-179640 (entire description)
Patent Document 3: Japanese Patent Application Laid-Open Publication No. 57-104884 (entire description)
Patent Document 4: Japanese Patent Application Laid-Open Publication No. 1-161081 (entire description)
Patent Document 5: Japanese Patent Application Laid-Open Publication No. 2-290591 (entire description)
Patent Document 6: Japanese Patent Publication No. 2-033385 (entire description)
Patent Document 7: Japanese Patent Application Laid-Open Publication No. 2003-192908 (entire description)
Patent Document 8: Japanese Utility Model Publication No. 5-15089 (entire description)